The present invention relates to flexible wing-sails and to wind-propelled vehicles including flexible wing-sails. The invention is especially useful in wind-propelled water vehicles, and is therefore described below with respect to such application, but it will be appreciated that the invention could also be used in other applications, such as for propelling vehicles over land or ice.
The conventional water vehicle sail functions in most wind directions like a wing of lower aerodynamic efficiency than an aircraft wing because the sail is a single sheet of fabric and also because the sail form is dictated by the vehicle structure. The conventional sail includes a fixed mast, fixed points at which the sail is anchored to the vehicle, and a single-surface structure. These features do not enable such a sail to have high aerodynamic efficiency, but rather result in its having a lower lift/drag ratio than that of an aircraft wing.
In recent years, a number of wing-sails have been proposed for use in water vehicles in order to better exploit the wind forces for propelling the vehicle. Basically, a wing-sail includes two curved surfaces defining a wing which is relatively thick and rounded at its leading edge, and tapers in thickness to its trailing edge. When the rounded, leading edge of an asymmetrical wing-sail is oriented to face the wind, the difference in air pressure between its two curved surfaces creates a lifting force which, in the case of wind-driven vehicles, is translated to a forward propulsion force. Examples of various constructions of wing-sails heretofore proposed are described in U.S. Pat. Nos. 4,685,410; 4,733,624; 4,856,449; 4,895,091; 5,406,902; 5,575,233; 5,622,131; and 6,141,809, and in U.K. Patents 2,008,514; and 2,196,310.
However, the proposed solutions to the problem generally were partial only. They included the option of a rotating mast that carries with it the usual rigging and sails as well as a rigid wing that cannot be reefed. Where a soft wing-sail was proposed permitting reefing, the wing-sail had an airfoil shape which is symmetric, wholly or partly and therefore was not sufficiently efficient. Several suggested solutions proposed an asymmetric airfoil, but the departures from symmetry are limited to a movable surface in the rear part of the wing-sail. Other solutions that were suggested are limited in flexibility, or are so complex that it is doubtful whether they could function under marine conditions. In most suggested solutions, the asymmetric variations are limited to two positions only (port/starboard), without control of the curvature of the airfoil shape of the wing-sail.
There is therefore a definite need for a flexible wing-sail construction providing increased aerodynamic efficiency, capable of being reefed and taken down, of being pointed to the wind, of enabling changes of the airfoil shape to either port or starboard in accordance with the apparent wind direction, and of enabling changes in the rate of asymmetry of the airfoil shape in accordance with the apparent windforce. Such higher efficiency would enable the attainment of higher speeds of travel, or alternatively, a reduction in the size of the sails, rigging and keel. It would also enable sailing a vessel more closely to the wind, less heeling, and more convenience in operating wind-driven vehicles, not only water vehicles, but also land and ice vehicles.